Modes of Excretion
1) Ammonotelism - The mode of excretion in which elimination of nitrogenous waste (excretory product) is mainly in the form of ammonia is called ammonotelism. Ammonia is highly toxic and water-soluble. It requires a large amount of water for elimination. The animals that follow this mode of excretion are known as ammonotelic animals. Example - Aquatic animals like bony fishes, aquatic amphibians, etc.
2) Ureotelism - The mode of excretion in which elimination of nitrogenous waste is in the form of urea is called ureotelism. Urea is comparatively less toxic to the body. Hence it can be stored in the body for a short period of time before it is excreted. It requires less amount of water for getting eliminated. The animals that follow this mode of excretion are known as ureotelic animals. Example - Humans, turtles, frogs, sharks etc.
3) Uricotelism - The mode of excretion in which elimination of nitrogenous waste is in the form of uric acid is called uricotelism. The animals that follow this mode of excretion are known as uricotelic animals. Most of these animals live in dry regions or do not consume plenty of water (eg. birds), hence they have to conserve water in their bodies. Uric acid is water-insoluble crystals that require a very less amount of water to get eliminated from the body. Example - Birds (class Aves), Helix (commonly known as land snails), cockroaches, lizards, snakes, etc.